Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen along with a C.sub.6 + non-aromatic byproduct. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function.
Since the product stream from the dehydrocyclodimerization process contains a mixture of compounds, it must undergo several separation steps in order to obtain usable products. An initial fractionation will separate any unreacted feedstream from a C.sub.6 + product stream. The unreacted feedstream is recycled to the dehydrocyclodimerization zone while the product stream is fed to a second fractionation column to separate the product mixture into benzene, toluene, xylenes and heavier aromatics. Two references present schemes for optimizing the yield of aromatics from a dehydrocyclodimerization process. The first of these is U.S. Pat. No. 4,642,402 which discloses that the product stream from a dehydrocyclodimerization process is first processed to give a light gas stream which is recycled to the dehydrocyclodimerization zone and a liquid phase product stream which is flowed to a first fractionation column to remove C.sub.3 -C.sub.5 hydrocarbons and provide a bottoms fraction which is high in benzene, toluene and xylenes. This bottoms fraction is now flowed through another fractionator which separates the benzene from the toluene and higher aromatics. A portion of the benzene is recycled to the dehydrocyclodimerization reaction zone to increase the yield of alkyl aromatics while the toluene and higher aromatic stream is further fractionated to give a toluene stream and a C.sub.8 + aromatic stream.
U.S. Pat. No. 4,806,700 also discloses a process scheme for enhancing the purity of the aromatics which uses a hydrodealkylation zone. This reference discloses that the product stream from the dehydrocyclodimerization zone is flowed to a fractionator which separates a benzene stream from a bottoms stream which contains toluene and higher aromatics. The bottoms stream is flowed to a hydrodealkylation zone to dealkylate the alkyl chains on the aromatics and provide a benzene product as well as a C.sub.1 -C.sub.2 light hydrocarbon stream which is vented. The benzene-containing product stream from the hydrodealkylation zone is further treated through a fractionation column to separate the benzene from the heavier aromatics.
Applicants have found that the benzene product from a dehydrocyclodimerization unit contains excessive amounts of non-aromatic C.sub.6 and C.sub.7 hydrocarbons which make it unsuitable for use in some petrochemical processes such as styrene or cyclohexane production. Neither of the process schemes described in the '402 or '700 references are suitable to provide a high purity benzene product since the '402 reference is aimed at providing a higher yield of toluene, while the '700 reference tries to increase the benzene yield by dealkylating alkyl aromatic compounds. Recognizing this need in the art, applicants have developed a process to give a benzene product that has a freezing point greater than or equal to 5.45.degree. C. making it suitable for use in styrene or cyclohexane production.
Applicants take the product stream from the dehydrocyclodimerization zone and flow it to a fractionation zone which is operated at conditions such that the majority of the C.sub.6 and C.sub.7 non-aromatic hydrocarbons along with a portion of the benzene product is removed via an overhead stream. The bottoms product stream from this fractionation zone will contain benzene, toluene and xylenes. The overhead process stream from this first fractionation zone is flowed to a conversion zone, along with a hydrogen-rich gas, where the non-aromatic hydrocarbons are converted to light (C.sub.1 -C.sub.2) hydrocarbons and a benzene fraction. The benzene fraction is combined with the bottoms fraction from the first fractionation zone and flowed to a second fractionation zone while the light hydrocarbons are vented. In this second fractionation zone the benzene is separated from the toluene, xylenes and heavier aromatics which in turn can be separated from each other in a third fractionation zone. An overhead stream can also be taken from this second fractionation zone which will contain some C.sub.6 and C.sub.7 non-aromatics along with some benzene and the second overhead process stream can also be flowed to the conversion zone to convert the non-aromatic hydrocarbons to light hydrocarbons and benzene.